Object Oriented Programming Languages

1. Object Oriented Programming Languages

2. What are the benefits? • Systems more adaptive to change • Information hiding • Encapsulation • Appeals to human cognition • Reuse • Gamma et al.: Design Patterns • Methodologies for design and analysis • Grady Booch: Object Oriented Design & Analysis • CASE tools

3. What is an OOPL? • No consensus • Cardelli + Wegner: OO = ADTs + first class objects + types inheriting attributes from a supertype • Finkel: OO = encapsulation + inheritance + overloading

4. Common features • Classes • Objects (instantiation) • (Single) Inheritance + redefinition • Type inheritance • Multiple inheritance • Method overloading • Dynamic lookup • Deferred implementations • Protection

5. Other features • Static typing, Genericity - Unit 8 • Exception handling, Garbage collection, Design by contract - Unit 9 • Multiple polymorphism • Reflection + Meta-object protocols • Packages • Class methods • Iterators • Operator overloading

6. Classes • A class is “a software element that describes an ADT and its partial or total definition” (Meyer) • Can be viewed as a set of members (or slots), which can be: • Data (attributes or fields) • Operations (methods, messages)

7. C++ Example: class FixedIntStack { int index, *arr; public: FixedIntStack(int size) { index = 0; arr = new int[size]; } ~FixedIntStack() { delete arr; } void push(int item){ arr[index++] = item; } int pop() { return arr[--index]; } };

8. Instantiation • Class instances are called objects, and are first class entities. • Constructors run at instantiation time. • Arguments can be passed to instantiations. • Instantiation is usually handled by an operator • ! In Eiffel • new in C++ and Java • Deallocation may call a destructor (or finalization method)

9. Instantiation Example C++ FixedIntStack *stack1 = new FixedIntStack(10); FixedIntStack stack2(10); stack1->push(5); stack2.push(5); Java FixedIntStack myStack = new FixedIntStack(10); myStack.push(5); A trend: objects PBR, primitive types by value

10. Inheritance • Incremental program extension class Integer inherit Number • Number is the parent of Integer • Integer is the child or descendant of Number • multiple inheritanceclass Mobile_Home inherit Vehicle, House

11. Subclassing ¹ subtyping ¹ is a • Subclassing - code & data sharing • Subtyping - type sharing • substitutability - a subtype may stand in for any parent type • polymorphism - void print(Object ob) • Specialization (is a) - implies subtyping and subclassing

12. Java Example class Rectangle { // default: extends Object float width, height; void set_width(float w) { width = w; } void set_height(float h) { height = h; } } class Square extends Rectangle { // These are overridden (redefined) methods: void set_width(float w) { super.set_width(w); super.set_height(w); } void set_height(float w) { set_width(float w); }

13. Method lookup (binding) Square s = new Square(); s.set_width(12); // Meaning is obvious Rectangle r = s; // substitutability! r.set_width(12); • Dynamic lookup: Square’s method is called. • Static lookup: Rectangle’s method is called • Java: Dynamic Lookup • C++: virtual keyword • Liskov substitution principle

14. Multiple Inheritance IOStream IStream OStream More from John Viega next Tuesday...

15. Protection • A way to enforce encapsulation. • C++: • no change by default • private, public, protected members • private, public, protected inheritance • break the rules with friend class X : protected Y, private Z { friendclass Q; public: X(); private: int datum; }

16. Protection • Eiffel: • No change by default • secret + non-secret members • selectively export operations to classes class MySecrets inherit GovtSecrets export {NONE} really_big_secret {ANY} common_knowledge feature {NSA, CIA, FBI} big_secret : Secret end